High strength steel material for oil well, excellent in sulfide stress cracking resistance, and production method thereof

ABSTRACT

This invention provides a high-strength steel material for an oil well, excellent in strength and SSC resistance, and having a yield strength of at least 120 ksi. The steel material contains C: 0.10 to 0.40%, Si≦0.5%, Mn≦0.5%, P≦0.015%, S≦0.0050%, Mo: 0.5 to 2.5%, Al: 0.005 to 0.1%, Ti: 0.005 to 0.1% and at least 3.4 times N, Nb: 0.01 to 0.1%, N≦0.01% and B: 0.0005 to 0.0050%, wherein the yield strength expressed by ksi and the Mo content satisfy the following relation (1), the balance of the C, Mn and Mo contents satisfies the following relation (2), and the steel material may contain, whenever necessary, at least one of Cr≦0.2%, W≦0.5%, V: 0.01 to 0.3%, Zr: 0.001 to 0.01%, Ca: 0.001 to 0.01%, Mg: 0.001 to 0.01% and REM: 0.001 to 0.01%:  
     α=Mo−0.15YS ≧−18.9  (1)  
     β=2.7C+Mn+2Mo≧2.0  (2).

TECHNICAL FIELD

[0001] This invention relates to a high-strength steel material such as a steel pipe, for example, casing, tubing, drill pipe, for an oil well, excellent in both strength and sulfide stress cracking (hereinafter referred to as “SSC”) resistance, that is used in oil and gas wells while it is exposed to a sour gas environment.

BACKGROUND ART

[0002] A steel material used for oil and gas wells containing hydrogen sulfide is required to possess SSC resistance. The true nature of this SSC is hydrogen embrittlement. Hydrogen embrittlement is more likely to occur as the strength of the steel material becomes higher. It has been therefore difficult to simultaneously satisfy the high strength and the high SSC resistance requirements.

[0003] With such a background, technical development has been done to obtain steel materials excellent in the SSC resistance and having high yield strength. For example, Japanese Examined Patent Publication (Kokoku) No.6-104849 describes a technology that achieves high strength, while SSC resistance is secured, by optimizing the steel components and the microstructure. However, the upper limit of the strength level that can be achieved by this technology presumably remains at a yield strength level of 120 ksi. Japanese Unexamined Patent Publication (Kokai) No.4-66645 discloses a technology that particularly addresses the interaction between Cr and Ni, and secures SSC performance and toughness with a higher strength by controlling the contents of these components. Nonetheless, the strength level that can be achieved by this technology is also, presumably, at the yield strength level of 120 ksi.

[0004] Market needs for oil well pipes have required, in recent years, a new type of steel material that has a yield strength of higher than about 125 ksi while it provides a sufficient SSC resistance. Demands for such a steel material are expected to grow in future.

[0005] Therefore, the yield strength of 120 ksi, that is, the level that has been achieved so for, is not sufficient to cope with the future needs, and development of a new type of steel is necessary.

DISCLOSURE OF THE INVENTION

[0006] In view of the background described above, the present invention aims at providing a steel material having an excellent SSC resistance with a high strength of at least 120 ksi in terms of the yield strength, that has been difficult to achieve in the past.

[0007] The inventor of the present invention conducted a series of studies to solve the problems described above. As a result, the present inventor has acquired the knowledge necessary and sufficient to constitute the present invention.

[0008] When the strength of the steel material becomes high, SSC occurs from the grain boundary. To suppress such a rupture, the microstructure must first be uniform. To acquire a high strength of at least 120 ksi and a high SSC resistance, there is no other selection but to use tempered martensite as the microstructure, and this structure, must be as uniform as possible.

[0009] If any heterogeneous phase having different characteristics is contained in the microstructure, the boundary of this heterogeneous phase or the heterogeneous phase itself functions as a breaking start point, and a sufficient SSC resistance cannot be obtained. Uniformity of this structure is substantially determined by the condition of quenching. In other words whether or not the heterogeneous phase develops depends on whether or not a sufficient and uniform quenched martensite structure is obtained throughout the steel material. Needless to say, a complete full martensitic microstructure is preferred. In consideration of thick steel materials or the limitation of the contents of elements capable of contributing to hardenability, that will be described later, the present inventor has examined the condition that is essential to the high strength level to which the present invention is directed. FIG. 1 shows the result.

[0010] In FIG. 1, a 25 mm-thick sheet material is subjected to quenching with water cooling from the austenite temperature of 900 to 930° C. Next, the hardness at the center portion of the thickness, that is, the most distant from the quenched end of the steel material, and the hardness of the portion just below the material surface that is just below the quenched end are measured. Hardenability is evaluated in terms of the ratio of the hardness of the former to that of the latter. At the same chance, a test specimen is machined from the center portion of the thickness to evaluate the SSC resistance.

[0011] The present inventor has acquired from FIG. 1 the knowledge that, when the ratio of the hardness of the center portion of the thickness to the hardness just below the plate surface is at least 95%, sufficient SSC resistance can be obtained even at a high strength of 120 ksi or more. The present inventor has also acquired the knowledge that, in order to sufficiently harden a steel material having a thickness of about 25 mm, an index β (2.7C+Mn+2Mo) calculated from the C, Mn and Mo contents must be at least 2.0.

[0012] The present inventor has examined the relationship between the strength that governs SSC performance, and the contents of the alloy elements, on the premise of the uniform microstructure without the heterogeneous phase, and has discovered the results shown in FIG. 2, wherein the relationship between Mo content and yield strength YS of the steel material after tempering is plotted. The contents of Mn and P affecting SSC performance was controlled to the lowest level that can be accomplished industrially. The relationship can be shown extremely clearly. The present inventor has found that to secure sufficient SSC resistance in a high strength material of YS≧120 ksi, the Mo content must be at least 0.5%, and the α value expressed by the relation with the yield strength YS, that is, α=Mo (wt %)−0.15YS (ksi), must be at least −18.9.

[0013] Furthermore, the present inventor has examined in detail the influences of Mn and Mo after P is set to a low level. As a result, the present inventor has found that both components interact as shown in FIG. 4. In the region where the Mn content is 0.3% or below, the SSC resistance depends solely on the Mo content. Within the range of the Mn content of 0.3 to 0.5%, however, the Mo content must be increased so as to correspond to the increase of the Mn content. When the Mn content exceeds 0.5%, the SSC resistance cannot be improved and further even when the Mo content is increased. Therefore, in order to make the most of the utility of Mo, the Mn content must be limited to 0.5% or below, and preferably to less than 0.3%.

[0014] The present invention has been completed on the basis of the observations described above. The gist of the present invention resides in the following points.

[0015] (1) A high-strength steel material for an oil well, excellent in sulfide stress cracking resistance, containing, in terms of mass %: C: 0.10 to 0.40%, Si≦0.5%, Mn≦0.5%, P≦0.015%, S≦0.0050%, Mo: 0.5 to 2.5%, Al: 0.005 to 0.1%, Ti: 0.005 to 0.1% and at least 3.4 times N%, Nb: 0.01 to 0.1%, N≦0.01% and B: 0.0005 to 0.0050%,

[0016] wherein the yield strength YS expressed by ksi and the Mo content satisfy the following relation (1) and the balance of the C, Mn and Mo contents satisfies the following relation (2).

α=Mo−0.15YS≧−18.9  (1)

β=2.7C+Mn+2Mo≧2.0  (2)

[0017] (2) A high-strength steel material for an oil, excellent in sulfide stress cracking resistance, containing, in terms of mass %: C: 0.10 to 0.40%, Si≦0.5%, Mn≦0.5%, P≦0.015%, S≦0.0050%, Mo: 0.5 to 2.5%, Al: 0.005 to 0.1%, Ti: 0.005 to 0.1% and at least 3.4 times N%, Nb: 0.01 to 0.1%, N≦0.01% and B: 0.0005 to 0.0050%,

[0018] wherein the yield strength YS expressed by ksi and the Mo content satisfy the following relation (1) and the balance of C, Mn and Mo contents satisfies the following relation (2); and

[0019] wherein the steel material contains further at least one of the following elements: W≦0.5%, V: 0.01 to 0.3%, Zr: 0.001 to 0.010%, Ca: 0.001 to 0.01%, Mg: 0.001 to 0.01% and REM: 0.001 to 0.01%.

α=Mo−0.15YS≧−18.9  (1)

β=2.7C+Mn+2Mo≧2.0  (2)

[0020] (3) A high-strength steel material for an oil well, excellent in sulfide stress cracking resistance, containing, in terms of mass %: C: 0.10 to 0.40%, Si≦0.5%, Mn<0.3%, P≦0.015%, S≦0.0050%, Mo: 0.5 to 2.5%, Al: 0.005 to 0.1%, Ti: 0.005 to 0.1% and at least 3.4 times N%, Nb: 0.01 to 0.1%, N≦0.01% and B: 0.0005 to 0.0050%,

[0021] wherein the yield strength YS expressed by ksi and the Mo content satisfy the following relation (1) and the balance of the C, Mn and Mo contents satisfies the following relation (2).

α=Mo−0.15YS≧−18.9  (1)

β=2.7C+Mn+2Mo≧2.0  (2)

[0022] (4) A high-strength steel material for an oil well, excellent in sulfide stress cracking resistance, containing, in terms of mass %: C: 0.10 to 0.40%, Si≦0.5%, Mn<0.3%, P≦0.015%, S≦0.0050%, Mo: 0.5 to 2.5%, Al: 0.005 to 0.1%, Ti: 0.005 to 0.1% and at least 3.4 times N%, Nb: 0.01 to 0.1%, N≦0.01% and B: 0.0005 to 0.0050%,

[0023] wherein the yield strength YS expressed by ksi and the Mo content satisfy the following relation (1) and the balance of the C, Mn and Mo contents satisfies the following relation (2); and

[0024] wherein said steel material further contains at least-one of the following elements: W≦0.5%, V: 0.01 to 0.3%, Zr: 0.001 to 0.010%, Ca: 0.001 to 0.01%, Mg: 0.001 to 0.01% and REM: 0.001 to 0.01%.

α=Mo−0.15YS≧−18.9  (1)

β=2.7C+Mn+2Mo≧2.0  (2)

[0025] (5) A high-strength steel material for an oil well, excellent in sulfide stress cracking resistance and having a yield strength of at least 120 ksi, containing, in terms of mass %: C: 0.10 to 0.40%, Si≦0.5%, Mn<0.5%, P≦0.015%, S≦0.0050%, Mo: 0.5 to 2.5%, Al: 0.005 to 0.1%, Ti: 0.005 to 0.1% and at least 3.4 times N%, Nb: 0.01 to 0.1%, N≦0.01% and B: 0.0005 to 0.0050%,

[0026] wherein the yield strength YS expressed by ksi and the Mo content satisfy the following relation (1) and the balance of the C, Mn and Mo contents satisfies the following relation (2).

α=Mo−0.15YS≧−18.9  (1)

β=2.7C+Mn+2Mo≧2.0  (2)

[0027] (6) A high-strength steel material for an oil well, excellent in sulfide stress cracking resistance and having a yield strength of at least 120 ksi, containing, in terms of mass %: C: 0.10 to 0.40%, Si≦0.5%, Mn<0.5%, P≦0.015%, S≦0.0050%, Mo: 0.5 to 2.5%, Al: 0.005 to 0.1%, Ti: 0.005 to 0.1% and at least 3.4 times N%, Nb: 0.01 to 0.1%, N≦0.01% and B: 0.0005 to 0.0050%,

[0028] wherein the yield strength YS expressed by ksi and the Mo content satisfy the following relation (1) and the balance of the C, Mn and Mo contents satisfies the following relation (2); and

[0029] wherein said steel material further contains at least one of the following elements: W≦0.5%, V: 0.01 to 0.3%, Zr: 0.001 to 0.010%, Ca: 0.001 to 0.01%, Mg: 0.001 to 0.01% and REM: 0.001 to 0.01%.

α=Mo−0.15YS≧−18.9  (1)

β=2.7C+Mn+2Mo≧2.0  (2)

[0030] (7) A high-strength steel material for an oil well, excellent in sulfide stress cracking resistance and having a yield strength of at least 120 ksi, containing, in terms of mass %: C: 0.10 to 0.40%, Si≦0.5%, Mn<0.3%, P≦0.015%, S≦0.0050%, Mo: 1.0 to 2.5%, Al: 0.0050 to 0.1%, Ti: 0.005 to 0.1% and at least 3.4 times N%, Nb: 0.01 to 0.1%, N≦0.01% and B: 0.0005 to 0.0050%,

[0031] wherein the yield strength YS expressed by ksi and the Mo content satisfy the following relation (1) and the balance of the C, Mn and Mo contents satisfies the following relation (2).

α=Mo−0.15YS≧−18.9  (1)

β=2.7C+Mn+2Mo≧2.0  (2)

[0032] (8) A high-strength steel material for an oil well, excellent in sulfide stress cracking resistance and having a yield strength of at least 120 ksi, containing, in terms of mass %: C: 0.10 to 0.40%, Si≦0.5%, Mn<0.3%, P≦0.015%, S≦0.0050%, Mo: 1.0 to 2.5%, Al: 0.005 to 0.1%, Ti: 0.0050 to 0.1% and at least 3.4 times N%, Nb: 0.01 to 0.1%, N≦0.01% and B: 0.0005 to 0.0050%,

[0033] wherein the yield strength YS expressed by ksi and the Mo content satisfy the following relation (1) and the balance of the C, Mn and Mo contents satisfies the following relation (2); and

[0034] wherein said steel material further contains at least one of the following elements: W≦0.5%, V: 0.01 to 0.3%, Zr: 0.001 to 0.010%, Ca: 0.001 to 0.01%, Mg: 0.001 to 0.01% and REM: 0.001 to 0.01%.

α=Mo−0.15YS≧−18.9  (1)

β=2.7C+Mn+2Mo≧2.0  (2)

[0035] (9) A method of producing a high-strength steel material for an oil well, excellent in sulfide stress cracking resistance, said method comprising the steps of hot processing the steel containing the components according to any of (1) through (8) and having the balance of the C, Mn and Mo contents satisfying the following relation (2);

[0036] heating for austenitizing the steel material to a temperature within the range of [Ac₃ point+20° C.] to 1,000° C.;

[0037] quenching the steel material to obtain a microstructure in which the hardness as-quenched at the remotest position in the steel from the cooled surface thereof is at least 95% of the hardness of the cooled surface; and

[0038] tempering the steel material at a temperature of 620 to 720° C.

β=2.7C+Mn+2Mo≧2.0  (2)

[0039] (10) A high-strength steel material for an oil well, excellent in sulfide stress cracking, according to any of (1) through (9), wherein a threshold stress for crack occurrence determined by a constant load type sulfide stress cracking test according to NACE TM0177-A is at least 80% of the yield strength.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040]FIG. 1 is a graph showing a ratio of the hardness at the center portion of the thickness in the as-quenched state to the hardness of the portion immediately below the surface layer, by a hardenability index β, and showing also the relationship between the SSC resistance of the steel material after quenching and β.

[0041]FIG. 2 is a graph showing the SSC resistance of a steel having high hardenability by the relationship between the Mo content and YS.

[0042]FIG. 4 is a graph showing the SSC resistance of a steel having an equivalent YS in association with the Mn and Mo contents.

[0043]FIG. 5 is a graph showing the SSC resistance in terms of the ratio of a threshold stress for crack ocurrence to YS as an index of the SSC resistance when YS changes.

BEST MODE FOR CARRYING OUT THE INVENTION

[0044] Hereinafter, the present invention will be explained in detail.

[0045] First, the reasons for limitation of the alloy components in the present invention will be explained. The content of each component is expressed in percent by weight.

[0046] C: Carbon is an essential element for securing simultaneously the intended high strength and the SSC resistance. The strength and the SSC resistance depend on hardenability. When the C content is less than 0.10%, quenching becomes incomplete and the strength drops. Even if required strength can be secured by adjusting the tempering condition, a sufficient SSC resistance cannot be acquired. When the C content exceeds 0.40%, on the other hand, the SSC resistance gets into saturation and susceptibility to quench crack and delayed fracture increases. Therefore, an appropriate range of the C content is set to the range of 0.10 to 0.40%.

[0047] Si: Silicon exists as a residue of a deoxidizer used in the steel making process. When the Si content exceeds 0.5%, the steel becomes brittle and the SSC resistance deteriorates. Therefore, the upper limit is set to 0.5%.

[0048] Mn: Manganese is an element detrimental to the SSC resistance and should not be added. However, since Mn has the function of improving hardenability, up to 0.5% as the upper limit of Mn may be contained in case that the C and Mo contents for improving hardenability are small and hardenability is not sufficient. When Mn is contained in the amount exceeding 0.5%, a satisfactory SSC resistance cannot be obtained even when complete quenching is conducted. Therefore the upper limit is set to 0.5%. Incidentally the preferred content of Mn is less than 0.3%.

[0049] P: Phosphorus is an impurity element which deteriorates the SSC resistance as it segregates in the grain boundary. Therefore, the P content must be restricted to as low as possible. The upper limit of the P content is set to 0.015% as an allowable level that can be attained stably according to existing refining technologies in view of the cost.

[0050] S: Sulfur is also an element that segregates in the grain boundary and deteriorates the SSC resistance. It is a basic requirement in the present invention to inhibit the Mn content that fixes S. Therefore, the S content should be reduced as low as possible. Since remarkable deterioration of the SSC resistance is not observed at a S content of less than 0.0050%, the upper limit is set to 0.0050%.

[0051] Mo: Molybdenum is one of the essential elements in the present invention. It is the element that restricts the grain boundary segregation of P and improves the temper softening resistance. Therefore, Mo is a suitable element for obtaining the high strength. At least 0.5% of Mo must be contained in order to secure a sufficient SSC resistance in the high strength region of YS≧120 ksi as shown in FIG. 2. The higher YS, the greater the Mo amount that is to be contained. The preferred range is at least 1.0%. When Mo is contained in a greater amount, the effect gets into saturation, and freedom of strength adjustment becomes small. Therefore, the upper limit is set to 2.5%.

[0052] Al: Aluminum is necessary for sufficiently deoxidizing the steel during the steel making process, and at least 0.005% of Al should be contained. When a greater amount of Al is contained, however, the amounts of alumina type inclusions increase with the result that the susceptibility to SSC is likely to increase. Therefore, the upper limit is set to 0.1%.

[0053] Ti: Titanium is contained in order to let B, that will be described later, sufficiently exhibit its hardenability enhancing function. In other words, N must be fixed as TiN in advance in order to prevent precipitation of BN. For this purpose, at least 0.005%, and moreover, at least 3.4 times the N content, of Ti must be added. However, a greater Ti content promotes precipitation of TiN and enhances SSC susceptibility. Therefore, the upper limit is set to 0.1%.

[0054] Nb: Niobium is the element effective for improving the SSC resistance because it reduces the grain boundary segregation of P through its grain refining effect. Therefore, at least 0.01% of Nb is contained. However, even when a greater amount of Nb is contained, the grain refining effect gets into saturation. On the contrary, the SSC resistance drops due to the drop of the grain boundary strength resulting from coarsening of carbides. Hence, the upper limit is set to 0.1%.

[0055] N: Nitrogen is an impurity element that hinders the hardenability improving effect of B and should be restricted to as low as possible. The upper limit is set to 0.01% as the allowable level that can be attained industrially and stably by the existing refining technologies in view of the cost.

[0056] B: Boron is the element that remarkably improves hardenability, and is one of the essential elements in the present invention for securing hardenability. If the B content is less than 0.0005%, sufficient hardenability cannot be secured. Therefore, the lower limit is set to 0.0005%. If the B content exceeds 0.0050%, on the other hand, the hardenability improving effect gets into saturation, and precipitation of carbo-borides becomes remarkable and the SSC resistance gets deteriorated, on the contrary. Therefore, the upper limit is set to 0.0050%.

α=Mo−0.15YS:

[0057] When the value α, that is calculated as the function of the yield strength YS (ksi) and the Mo content (wt %), exceeds −18.9 as shown in FIG. 2, an excellent SSC resistance can be obtained. When it is below −18.9, a satisfactory SSC resistance cannot be obtained even when the individual components satisfy the respective conditions. For this reason, the present invention stipulates α≧−18.9 as the essential requirement of the invention in addition to the component conditions described above.

β=2.7C+Mn+2Mo:

[0058] In addition to the component conditions described above, the index β, that is calculated from the contents (wt %) of the alloy elements contributing to hardenability, such as C, Mn and Mo, is set to at least 2.0, as shown in FIG. 1, in order to secure sufficient hardenability. The upper limit is calculated as 6.11 from each of the upper limit contents of C, Mn and Mo.

[0059] In addition to the elements described above, the steel material of the present invention may contain at least one element selected from the following elements, whenever necessary.

[0060] W: Tungsten has the function of improving both hardenability and temper softening resistance. However, its effect is not sufficient if its content is less than 0.01%. When the content exceeds 0.5%, the effect gets into saturation. Therefore the suitable range of the W content is set to 0.01 to 0.5%.

[0061] V: Vanadium has the function of improving temper softening resistance. When its content is greater than 0.01%, it is effective for improving the strength. When an excessive amount of V is contained, however, the SSC resistance gets deteriorated. Therefore, the upper limit is set to 0.3%.

[0062] Zr: Zirconium has an effect of restricting grain boundary segregation of P. At least 0.001% of Zr must be contained in order to obtain the effect. Zr is an expensive element and, if contained too much, Zr increases the amounts of oxides and possibly increases the SSC susceptibility. Therefore, the upper limit is set to 0.010%.

[0063] Ca, Mg and REM: These elements mitigate the stress concentration by spheroidizing the morphology of inclusions, at the same time, mitigate the grain boundary segregation of S by fixing it. If the content of these elements is less than 0.001%, the effect is small. On the other hand, at an excessive content, the amounts of oxides increase and the SSC susceptibility is possibly enhanced. Therefore, the upper limit of these element is set to 0.010%.

[0064] In the present invention, the steel having the alloy composition described above is molten in a converter or an electric furnace and cast. It is then shaped by an ordinary hot rolling method into a desired shape such as a pipe, a sheet or rod, is subjected to quenching and tempering, and is refined to a desired strength.

[0065] The heat-treating condition in the present invention will be explained as follows.

[0066] The austenitizing temperature for quenching is at least [Ac₃ point+20° C.] and not higher than 1,000° C. If it is lower than Ac₃ point+20 ° C., austenitizing of the steel material is not sufficient and a uniform martensite structure cannot be obtained easily. When the austenitizing temperature exceeds 1,000° C., on the other hand, the grain growth becomes remarkable and the area of the grain boundary decreases. In consequence, the SSC resistance deteriorating effect of the segregation elements such as P becomes remarkable. Therefore, the suitable austenitizing temperature is within the range of [Ac₃+20° C.] to 1,000° C. Incidentally, the typical Ac₃ point of the steel material according to the present invention is 830° C.

[0067] As described above, further, the steel material quenched from this austenitizing temperature must have a uniform structure in order to acquire a satisfactory SSC resistance. It is necessary that the hardness of the portion that is most distant from the quenched end has a value of at least 95% in terms of the ratio to the hardness of the portion immediately below the quenched end. In the chemical composition of the present invention, expected hardenability can be secured even in a thick material having a thickness of about 25 mm.

[0068] A suitable range of the tempering temperature is 620 to 720° C. Under the low temperature condition of lower than 620° C., YS becomes excessively high and the value a described above becomes low. In consequence, the satisfactory SSC resistance cannot be obtained. When the tempering temperature exceeds 720° C., on the other hand, the temperature is in the dual phase region, and uniformity of the microstructure is lost with the result of an increase of the SSC susceptibility. Therefore, the range of 630 to 720° C. is set as the suitable condition for a tempering temperature.

[0069] The YS value can be secured within the predetermined range due to the heat treatment temperature range described above, for example, by controlling the quenching temperature, and the relation α=Mo−0.15YS≧−18.9 can be satisfied.

[0070] As described above, it is possible to obtain a high strength, SSC-resistant steel material satisfying YS 120 ksi, which has not been achieved in the past, by optimizing the microstructure and the components. When the present invention is applied to the strength region of YS of lower than 120 ksi, the SSC resistance ca also be improved. FIG. 5 shows the measurement result of the threshold stress for crack occurrence (σth) of a steel material, that satisfies the microstructure and the chemical compositions of the present invention and is refined to YS of 117 to 120 ksi, while the applied stress is varied by a NACE TM0177-A constant load SSC test. It can be thus said that the structural factors of the present invention improve not only the SSC resistance of the steel material having YS≧120 ksi but also that of the steel material having YS of less than 120 ksi due to the improvement of σth.

EXAMPLE

[0071] Each steel material having a chemical composition tabulated in Table 1 was molten in a vacuum melting furnace, and the resulting ingot was hot rolled to a plate having a thickness of 25 mm. The plate was heated to 900 to 1,025° C. for austenitizing, and was then subjected to quenching treatment by water. The hardness at the center of the thickness and the hardness of a portion just below the surface were measured. Hardenability was evaluated by calculating their ratio. Subsequently, tempering was conducted at 630 to 730° C. A round rod tensile test specimen was machined from the center of the thickness of this plate, and the tensile test was carried out.

[0072] A test specimen for a sulfide stress cracking test, which had a round rod shape having a length of 25 mm at parallel portions and a diameter of 6.2 mm and was stipulated in NACE-TM0177-A, was also machined from the center of the thickness. This test was carried out in a corrosive solution containing 0.5% acetic acid+5% NaCl and containing H₂S saturated at a partial pressure of 1 atm at 25° C., for 720 hours while a constant stress corresponding to 80% of the yield strength was loaded to the specimen. Additional tests were carried out by decreasing the applied stress step-wise by 5% YS, whenever necessary, for those specimens which underwent rupture at the applied stress of 80% YS, and by increasing the applied stress step-wise by 5% YS for those specimens which did not undergo rupture. In this way, a threshold stress for crack occurrence (σth) was determined.

[0073] Hardenability was evaluated as “good” when the hardness of the center portion of the thickness was greater than 95% of the hardness of the portion immediately below the surface. The SSC resistance was evaluated as “good” for those specimens which did not undergo rupture during the test for 720 hours. The test result was tabulated in Tables 2 and 3.

[0074] Nos. 1 to 11 and Nos. 101 to 104 represent the test results that fell within the range of the present invention. These specimens satisfied both high strength of YS 120 ksi and excellent SSC resistance.

[0075] On the other hand, Nos. 18 to 30 and Nos. 105 to 107 of Comparative Examples could not provide excellent SSC resistance because their components were outside the range of the present invention. The components of Nos. 12, 13, 14, 16 and 17 of Comparative Examples were within the range of the present invention but the α values were outside the range of the present invention. Nos. 15 and 31 of Comparative Examples could not provide sufficient SSC resistance because the tempering temperature and the austenitizing temperature were outside the range of the present invention, respectively.

[0076] In Nos. 108 and 109 tabulated in Table 3, YS was less than 120 ksi but σth was higher than that of Comparative Examples Nos. 113 and 114 having an equivalent YS value. Therefore, the SSC resistance was improved. Nos. 110 and 111 had the YS values higher than 120 ksi and their σth values, were as high as 85% of YS. On the other hand, in Comparative Example No. 112, YS remained at 85% of σth though YS was low. No. 115 had insufficient SSC resistance. TABLE 1 Chemical Compositions (wt %; * ppm) Item Symbol C Si Mn P* S* Mo Al Ti Nb N* B* Ti/N W V Zr* Ca* Mg* REM* β This X-1 0.20 0.15 0.06 80 15 1.30 0.025 0.021 0.026 39 12  5.38 3.20 Inven- X-2 0.20 0.16 0.46 81 15 1.30 0.026 0.022 0.031 41 13  5.37 3.60 tion X-3 0.27 0.15 0.15 70 25 0.58 0.029 0.031 0.019 55 13  5.64 2.04 X-4 0.29 0.15 0.04 60 10 1.51 0.025 0.026 0.026 39 12  6.67 0.21 3.84 A 0.29 0.15 0.45 81 10 0.90 0.040 0.035 0.029 70 15  5.00 0.25 0.02 21 3.03 B 0.20 0.15 0.33 70 25 0.62 0.029 0.031 0.019 55 13  5.64 2.11 C 0.20 0.15 0.33 70 15 0.89 0.029 0.031 0.019 55 13  5.64 2.65 D 0.20 0.20 0.13 81 25 1.40 0.036 0.075 0.021 39 7 19.23 3.47 E 0.19 0.25 0.08 86 16 1.80 0.038 0.021 0.031 41 38  5.12 15 15 4.19 F 0.12 0.15 0.23 57 9 2.21 0.047 0.029 0.041 51 11  5.69 4.97 G 0.19 0.20 0.05 75 10 1.50 0.035 0.041 0.031 45 21  9.11 21 12 3.56 Comp- Y-1 0.23 0.16 0.30 80 16 0.78 0.024 0.023 0.025 44 11  5.23 2.48 arative Y-2 0.25 0.15 0.13 80 16 0.42 0.025 0.025 0.025 38 12  6.58 1.65 Ex- Y-3 0.21 0.16 0.55 80 16 1.88 0.026 0.024 0.029 41 12  5.85 4.88 ample H 0.15 0.21 0.30 91 25 1.55 0.026 0.008 0.018 65 13  1.90 19 3.81 I 0.28 0.15 0.46 135 21 0.48 0.032 0.040 0.036 39 20 10.30 2.18 J 0.17 0.14 0.21 70 23 0.61 0.036 0.025 0.038 46 13  5.43 1.89 K 0.12 0.15 0.20 75 28 0.60 0.046 0.036 0.030 55 12  6.55 1.72 L 0.25 0.16 0.33 51 18 1.40 0.040 0.034 0.034 51 56  6.67 3.81 M 0.19 0.17 0.45 165 14 0.75 0.032 0.033 0.036 55 15  6.00 2.46 N 0.25 0.15 0.33 71 16 0.91 0.031 0.031 0.039 56 13  5.54 2.83 O 0.18 0.15 0.60 136 13 0.71 0.033 0.035 0.029 60 12  5.83 2.51 P 0.19 0.14 0.36 129 55 0.72 0.026 0.029 0.034 78 9  3.71 2.31 Q 0.23 0.15 0.33 71 16 0.91 0.031 0.031 0.039 56 13  5.54 2.77 R 0.21 0.18 0.31 120 20 1.19 0.034 0.021 0.111 40 13  5.25 3.26

[0077] TABLE 2 Heat Treatment Conditions, Strength and SSC Test Results SSC Test Results under Quenching Tempering conditions of Temperature Temperature YS load stress = Item No. Composition (° C.) Hardenability (° C.) (ksi) α 80% of YS This 101 X-1 950 Good 670 126 −17.90 Good Invention 102 X-2 930 Good 670 125 −17.45 Good 103 X-3 930 Good 670 123 −17.87 Good 104 X-4 930 Good 710 131 −18.14 Good 1 A 900 Good 680 123 −17.85 Good 2 B 900 Good 670 124 −17.98 Good 3 C 900 Good 660 125 −17.86 Good 4 C 930 Good 650 129 −18.46 Good 5 D 900 Good 670 129 −17.95 Good 6 E 900 Good 700 131 −17.85 Good 7 E 930 Good 680 136 −18.60 Good 8 E 900 Good 720 121 −16.35 Good 9 F 900 Good 690 135 −18.04 Good 10 F 930 Good 680 138 −18.49 Good 11 G 930 Good 670 131 −18.15 Good Comparative 105 Y-1 930 Good 650 124 −17.82 Poor Example 106 Y-2 930 Poor 650 123 −18.03 Poor 107 Y-3 930 Good 690 124 −16.72 Poor 12 C 900 Good 640 134 −19.21 Poor 13 C 900 Good 630 140 −20.11 Poor 14 E 900 Good 680 141 −19.35 Poor 15 E 900 Good 730 103 −15.27 Poor 16 G 930 Good 660 145 −20.25 Poor 17 G 900 Good 660 137 −19.05 Poor 18 H 900 Poor 650 129 −17.80 Poor 19 I 930 Good 690 125 −18.27 Poor 20 J 900 Poor 650 119 −17.24 Poor 21 K 900 Poor 650 115 −16.65 Poor 22 L 930 Good 700 131 −18.25 Poor 23 M 900 Good 660 128 −18.45 Poor 24 N 900 Good 650 129 −18.44 Poor 25 N 900 Good 670 118 −16.79 Good 26 O 930 Good 660 126 −18.19 Poor 27 P 900 Good 660 128 −18.19 Poor 28 Q 900 Good 650 125 −17.84 Poor 29 Q 900 Good 660 118 −16.79 Good 30 R 900 Good 670 130 −18.31 Poor 31 C 1025 Good 660 130 −18.61 Poor

[0078] TABLE 3 Heat Treatment Conditions, Strength and SSC Test Results Quenching Tempering Temperature Temperature YS σth Item No. Composition (° C.) Hardenability (° C.) (ksi) α (× YS) This 108 X-1 950 Good 690 111 −15.35 0.95 Invention 109 X-1 950 Good 680 119 −16.55 0.90 110 X-1 950 Good 670 126 −17.90 0.85 111 X-1 950 Good 660 132 −18.50 0.85 Comparative 112 Y-1 950 Good 690 106 −15.12 0.85 Example 113 Y-1 950 Good 680 112 −16.02 0.85 114 Y-1 950 Good 670 118 −16.92 0.80 115 Y-1 950 Good 660 121 −18.42 0.55

[0079] INDUSTRIAL APPLICABILITY

[0080] According to the present invention, as described above, a steel material for an oil well that satisfies both a high strength of at least 120 ksi in terms of a yield strength and excellent SSC resistance can be produced. 

1. A high-strength steel material, for an oil well, excellent in sulfide stress cracking resistance, containing, in terms of mass %: C: 0.10 to 0.40%, Si≦0.5%, Mn≦0.5%, P≦0.015%, S≦0.0050%, Mo: 0.5 to 2.5%, Al: 0.005 to 0.1%, Ti: 0.005 to 0.1% and at least 3.4 times N%, Nb: 0.01 to 0.1%, N≦0.01% and B: 0.0005 to 0.0050%,

wherein the yield strength YS expressed by ksi and the MO content satisfy the following relation (1) and the balance of the C, MN and MO contents satisfies the following relation (2): α=Mo−0.15YS≧−18.9  (1) β=2.7C+Mn+2Mo≧2.0  (2).
 2. A high-strength steel material, for an oil, excellent in sulfide stress cracking resistance, containing, in terms of mass %: C: 0.10 to 0.40%, Si≦0.5%, Mn≦0.5%, P≦0.015%, S≦0.0050%, Mo: 0.5 to 2.5%, Al: 0.005 to 0.1%, Ti: 0.005 to 0.1% and at least 3.4 times N%, Nb: 0.01 to 0.1%, N≦0.01% and B: 0.0005 to 0.0050%,

wherein the yield strength YS expressed by ksi and the Mo content satisfy the following relation (1) and the balance of C, Mn and Mo contents satisfies the following relation (2); α=Mo−0.15YS≧−18.9  (1) β=2.7C+Mn+2Mo≧2.0  (2), and wherein said steel material contains further at least one of the following elements: W≦0.5%, V: 0.01 to 0.3%, Zr: 0.001 to 0.010%, Ca: 0.001 to 0.01%, Mg: 0.001 to 0.01% and REM: 0.001 to 0.01%.


3. A high-strength steel material, for an oil well, excellent in sulfide stress cracking resistance, containing, in terms of mass %: C: 0.10 to 0.40%, Si≦0.5%, Mn<0.3%, P≦0.015%, S≦0.0050%, Mo: 0.5 to 2.5%, Al: 0.005 to 0.1%, Ti: 0.005 to 0.1% and at least 3.4 times N%, Nb: 0.01 to 0.1%, N≦0.01% and B: 0.0005 to 0.0050%,

wherein the yield strength YS expressed by ksi and the Mo content satisfy the following relation (1) and the balance of the C, Mn and Mo contents satisfies the following relation (2): α=Mo−0.15YS≧−18.9  (1) β=2.7C+Mn+2Mo≧2.0  (2).
 4. A high-strength steel material, for an oil well, excellent in sulfide stress cracking resistance, containing, in terms of mass %: C: 0.10 to 0.40%, Si≦0.5%, Mn<0.3%, P≦0.015%, S≦0.0050%, Mo: 0.5 to 2.5%, Al: 0.005 to 0.1%, Ti: 0.005 to 0.1% and at least 3.4 times N%, Nb: 0.01 to 0.1%, N≦0.01% and B: 0.0005 to 0.0050%,

wherein the yield strength YS expressed by ksi and the Mo content satisfy the following relation (1) and the balance of the C, Mn and Mo contents satisfies the following relation (2); and Wherein said steel material contains further at least one of the following elements: W≦0.5%, V: 0.01 to 0.3%, Zr: 0.001 to 0.010%, Ca: 0.001 to 0.01%, Mg: 0.001 to 0.01% and REM: 0.001 to 0.01%:

α=Mo−0.15YS≧−18.9  (1) β=2.7C+Mn+2Mo≧2.0  (2).
 5. A high-strength steel material, for an oil well, excellent in sulfide stress cracking resistance and having a yield strength of at least 120 ksi, containing, in terms of mass %: C: 0.10 to 0.40%, Si≦0.5%, Mn<0.5%, P≦0.015%, S≦0.0050%, Mo: 0.5 to 2.5%, Al: 0.005 to 0.1%, Ti: 0.005 to 0.1% and at least 3.4 times N%, Nb: 0.01 to 0.1%, N≦0.01% and B: 0.0005 to 0.0050%,

wherein the yield strength YS expressed by ksi and the Mo content satisfy the following relation (1) and the balance of the C, Mn and Mo contents satisfies the following relation (2). α=Mo−0.15YS≧−18.9  (1) β=2.7C+Mn+2Mo≧2.0  (2).
 6. A high-strength steel material, for an oil well, excellent in sulfide stress cracking resistance and having a yield strength of at least 120 ksi, containing, in terms of mass %: C: 0.10 to 0.40%, Si≦0.5%, Mn≦0.5%, P≦0.015%, S≦0.0050%, Mo: 0.5 to 2.5%, Al: 0.005 to 0.1%, Ti: 0.005 to 0.1% and at least 3.4 times N%, Nb: 0.01 to 0.1%, N≦0.01% and B: 0.0005 to 0.0050%,

wherein the yield strength YS expressed by ksi and the Mo content satisfy the following relation (1) and the balance of the C, Mn and Mo contents satisfies the following relation (2); and wherein said steel material further contains at least one of the following elements: W ≦ 0.5%, V: 0.01 to 0.3%, Zr: 0.001 to 0.010%, Ca: 0.001 to 0.01%, Mg: 0.001 to 0.01% and REM: 0.001 to 0.01%:

α=Mo−0.15YS≧−18.9  (1) β=2.7C+Mn+2Mo≧2.0  (2).
 7. A high-strength steel material for an oil well, excellent in sulfide stress cracking resistance and having a yield strength of at least 120 ksi, containing, in terms of mass %: C: 0.10 to 0.40%, Si ≦ 0.5%, Mn < 0.3%, P ≦ 0.015%, S ≦ 0.0050%, Mo: 1.0 to 2.5%, Al: 0.0050 to 0.1%, Ti: 0.005 to 0.1% and at least 3.4 times N%, Nb: 0.01 to 0.1%, N ≦ 0.01% and B: 0.0005 to 0.0050%,

wherein the yield strength YS expressed by ksi and the Mo content satisfy the following relation (1) and the balance of the C, Mn and Mo contents satisfies the following relation (2): α=Mo−0.15YS≧−18.9  (1) β=2.7C+Mn+2Mo≧2.0  (2).
 8. A high-strength steel material for an oil well, excellent in sulfide stress cracking resistance and having a yield strength of at least 120 ksi, containing, in terms of mass %: C: 0.10 to 0.40%, Si ≦ 0.5%, Mn < 0.3%, P ≦ 0.015%, S ≦ 0.0050%, Mo: 1.0 to 2.5%, Al: 0.005 to 0.1%, Ti: 0.0050 to 0.1% and at least 3.4 times N%, Nb: 0.01 to 0.1%, N ≦ 0.01% and B: 0.0005 to 0.0050%,

wherein the yield strength YS expressed by ksi and the Mo content satisfy the following relation (1) and the balance of the C, Mn and Mo contents satisfies the following relation (2); and wherein said steel material further contains at least one of the following elements: W ≦ 0.5%, V: 0.01 to 0.3%, Zr: 0.001 to 0.010%, Ca: 0.001 to 0.01%, Mg: 0.001 to 0.01% and REM: 0.001 to 0.01%:

α=Mo−0.15YS≧−18.9  (1) β=2.7C+Mn+2Mo≧2.0  (2).
 9. A method of producing a high-strength steel material, for an oil well, excellent in sulfide stress cracking resistance, said method comprising the steps of: hot processing said steel containing said components according to any of claims 1 through 8 and having the balance of the C, Mn and Mo contents satisfying the following relation (2); heating for austenitizing said steel material to a temperature within the range of [Ac₃ point+20° C.] to 1,000° C.; quenching said steel material to obtain a microstructure in which the hardness as-quenched at the remotest position of said steel material from the cooled surface thereof is at least 95% of the hardness of the cooled surface; and tempering said steel material at a temperature of 620 to 720° C.: β=2.7C+Mn+2Mo≧2.0  (2).
 10. A high-strength steel material for an oil well excellent in sulfide stress cracking, according to any of claims 1 through 9, wherein a threshold stress for crack occurrence determined by a constant load type sulfide stress cracking test according to NACE TM0177-A is at least 80% of the yield strength. 